Magnetite dissolution in siliceous sediments
Magnetite dissolution in siliceous sediments
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硅质沉积物中的磁铁矿溶解
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发表时间：
2003
期刊：
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通讯作者：
M. Palmer
中科院分区：
文献类型：
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作者：
F. Florindo;A. Roberts;M. Palmer
磁铁矿溶解和随之而来的磁化损失在还原沉积环境中被广泛观察到，其中Eh-pH值随深度的降低是由细菌介导的有机碳降解驱动的。我们已经观察到低磁化沉积物孔隙水二氧化硅浓度升高，所产生的成岩作用的生物硅和/或火山灰。磁化强度的这些损耗大于贫磁铁矿沉积物的稀释，表明磁铁矿的沉积后破坏已经发生。生物硅质沉积物通常还含有高浓度的有机碳，这使得很难将有机碳相关的磁铁矿溶解与其他可能的磁铁矿溶解机制分开。然而，我们所研究的沉积序列中磁铁矿溶解的程度与环境的氧化还原状态没有明显的关系。这表明，其他机制可能引起磁铁矿溶解在这些硅质沉积物。热力学计算表明，磁铁矿在溶解二氧化硅浓度升高的条件下（以及适当的Eh-pH条件下）是不稳定的，并预测磁铁矿将分解产生含铁蒙皂石。对分布广泛的大洋钻探计划地点的磁化率和孔隙水地球化学数据的调查支持了这种观察到的高溶解二氧化硅浓度和低磁化率之间的联系。这种观察到的联系也适用于低生物硅生产力（和低有机碳含量），但由于溶解的火山灰高间隙二氧化硅浓度的环境。因此，磁铁矿的溶解作用是硅质沉积环境的一个共同特征。
Magnetite dissolution, and consequent loss of magnetization, is widely observed in reducing sedimentary environments, where the decrease in Eh‐pH values with depth is driven by bacterially mediated degradation of organic carbon. We have observed low magnetizations in sediments with elevated pore water silica concentrations that arise from diagenesis of biogenic silica and/or silicic volcanic ash. These depletions in magnetization are greater than can be accounted for by dilution with magnetite‐poor sediments and suggest that postdepositional destruction of magnetite has occurred. Biosiliceous sediments usually also contain elevated concentrations of organic carbon, which makes it difficult to separate organic‐carbon‐related magnetite dissolution from other possible mechanisms for magnetite dissolution. However, the extent of magnetite dissolution in the sedimentary sequences that we have studied is not obviously related to the redox‐state of the environment. This suggests that other mechanisms might have given rise to magnetite dissolution in these siliceous sediments. Thermodynamic calculations indicate that magnetite is unstable under conditions of elevated dissolved silica concentrations (and appropriate Eh‐pH conditions) and predict that magnetite will break down to produce iron‐bearing smectite. A survey of magnetic susceptibility and pore water geochemical data from widely distributed Ocean Drilling Program sites supports this observed link between high dissolved silica concentrations and low magnetic susceptibilities. This observed link also holds for environments with low biogenic silica productivity (and low organic carbon content) but with high interstitial silica concentrations due to dissolution of silicic volcanic ashes. Dissolution of magnetite is therefore predicted to be a common feature of siliceous sedimentary environments.